The present invention relates to a process for the production of dihydroketoisophorone (DH-KIP; 2,2,6-trimethylcyclohexane-1,4-dione) by epoxidation of xcex2-isophorone with a percarboxylic acid solution in an inert, non-water-soluble solvent, which is, at the same time, the solvent used for epoxidation: of xcex2-isophorone (xcex2-IP) to xcex2-isophorone epoxide (xcex2-IPO) and subsequent isomerization to 4-hydroxyisophorone (HIP) and the product DH-KIP. In particular, an industrially advantageous process is provided, in which the reaction sequence: epoxidation, ring opening of the epoxide to HIP and isomerization of HIP to the product takes place in a pH range which allows the entire synthesis sequence to be carried out selectively without the need for frequent changes from acid to basic condition.
In particular, an advantageous process is described, in which all reactions can be carried out one after the other in one reaction unit, without the need for intermediate isolation of discharged intermediates.
The DH-KIP obtained from this process can be converted directly to trimethylhydroquinonediacetate (TMHQ-DA) by oxidative aromatization. Trimethylhydroquinonediacetate is a central educt of vitamin E acetate synthesis. DH-KIP is also an important component for various carotinoid syntheses. In addition to its uses in the human sphere, vitamin E acetate is used in the form of special formulations as an additive for animal feed.
3,5,5-trimethyl-4-hydroxy-cyclohex-2-en-1-one (HIP) is described in the literature as a flavoring and aromatizing substance (JP-81 35 990; CH 549 961; DE 22 02 066). Its se as a food flavoring is also known (CH 549 956; M. Ishikara et al., J.Org. Chem. 1986, 51, 491 et seq.). HIP also has a variety of applications as a synthetic component for natural products and various pharmaceuticals (N. S. Zarghami et al., Phytochemistry 1971, 10, 2755 et seq.; J. N. Marx and F. Sondheimer, Tetrahedron Lett., Suppl. No. 8, Pt 1, 1-7, 1966). In particular xcex2-IPO is an important intermediate for the synthesis of 2,6,6-trimethylcyclohexane-1,4-dione and thus for vitamin E. The conventional synthesis sequence is as follows: 
The known processes for the production of xcex2-IPO produce only unsatisfactory yields. It was found that oxidation of xcex2-isophorone normally proceeds to 4-oxo-isophorone, hydroxyisophorone being formed in concentrations of 1-50%, depending on the oxidizing agent used. The formation of hydroxyisophorone appears, according to the processes described, to be a secondary reaction. If the course of the reaction is followed, it becomes clear that hydroxyisophorone is not the intermediate product of 4-oxo-isophorone, as HIP is virtually inert under the oxidation conditions.
The epoxidation of xcex2-isophorone originates with Isler et.al. (Helv. Chim. Acta 39, 1956, 2041 et seq.), who carry out epoxidation with peracetic acid as the oxidizing agent in acetic acid as the solvent and, after changing the pH value to 8-9 with aqueous sodium hydroxide solution, isolate only unsatisfactory yields of HIP. The need to basify with dilute sodium hydroxide solution the solution first obtained, which contains xcex2-IPO, for the production of HIP, gives rise to a stoichiometric salt load (formation of sodium acetate) and prevents recycling of the organic acid. No details are given of the isomerization of xcex2-IP to alpha-isophorone which occurs as a secondary reaction. The yields of HIP from xcex2-IP according to this publication, amount to only about 60%, due to the formation of alpha-IP and the non-selective ring opening of xcex2-IPO to HIP.
The same procedure is described in British patent 791 953, although no details of yields and the formation of by-products are given here. U.S. Pat. No. 2,857,423 by the same authors gives an equally incomplete description of the production of DH-KIP. According to these publications DH-KIP is formed either from HIP by acid catalysis, HIP being produced in a separate reaction and isolated, or from ketoisophorone by partial hydrogenation of the double bond.
Zarghami et al. also (Phytochemistry 10, 1971, 2755 et seq.) do not disclose yields of xcex2-IP epoxide from their reaction with peracetic acid. Tetrahedron Lett. Suppl. No. 8, Pt. 1, 1966, 1-7 gives a further description of the epoxidation of xcex2-isophorone. Organic solvents such as chloroform, using meta-chlorobenzoic acid as the oxidizing agent, are described, m-chlorobenzoic acid being precipitated out from the solution after completion of the redox reaction and a product profile being produced which consists of xcex2-IP epoxide and HIP in a ratio of 1:1, and alpha-isophorone. It is obvious that, according to this process, neither the undesirable re-isomerization to alpha-IP nor the consecutive reaction to HIP can be suppressed. After hydrolysis at a basic pH, 87% HIP is isolated. This procedure is unsatisfactory as the pH environment must be changed several times to produce HIP, which entails a significant salt load and produces only moderate yields.
All of these processes have in common that they produce unsatisfactory yields of xcex2-isophorone epoxide, due to the non-selective reaction process or unsuitable oxidizing agent, or to the presence of water in the reaction medium, which both catalyzes the reverse reaction of xcex2-isophorone and destabilizes the epoxide. The formation of the diol can also be detected from the xcex2-IP epoxide as a result of the accumulation of water.
A further reaction, observed when the reaction is not sufficiently controlled, is the epoxidation of alpha-IP (which is formed xe2x80x9cin situxe2x80x9d from xcex2-IP by isomerization) to alpha-IP epoxide, and its consecutive reaction of isomerization to 2-hydroxyisophorone. These principal secondary reactions are observed also in epoxidation with other substrates, the diols and hydroxyesters being obtained in particular (see W. M. Weigert, Wasserstoffperoxid und seine Derivate [Hydrogen peroxide and its derivatives], Hxc3xcthig Verlag Heidelberg 1978, page 79 et seq.).
The following diagram shows the possible secondary and consecutive reactions of xcex2-IP epoxidation: 
The epoxidation of xcex2-isophorone in the presence of anhydrous peroxidation reagents such as alkylhydroperoxides is also described (Hutter, Baiker et al., Journal Mol. Cat. 172, 427-435, 1997). A heterogenous contact SiO2xe2x80x94TiO2 mixed oxide activates the peroxide, expensive pre-treatment of the catalyst, or the addition of further auxiliary substances such as bases, being necessary to achieve higher selectivities, partly in order to suppress the formation of HIP. Although this process has achieved the best epoxide selectivities hitherto, it is not advantageous to use alkylhydroperoxides, which are spent stoichiometrically, for an industrial process. It is also undesirable to use a heterogeneous contact, which is costly to prepare.
DP 38 06 835 describes the oxidation of xcex2-IP to HIP by reaction with aqueous hydrogen peroxide in the presence of formic acid. xcex2-IP epoxide is discharged as an intermediate, but a re-isomerization rate in the range 20-35% make the process unattractive from an industrial point of view.
No satisfactory process is described, in particular for the rearrangement of xcex2-isophorone epoxide or a mixture of the xcex2-IPO first obtained by epoxidation and hydroxyisophorone. The process suggested in British patent 791 953 for the rearrangement of HIP to DH-KIP is laborious, as HIP must first be produced from xcex2-IPO by basic hydrolysis in a separate process step. The consecutive reaction with strong acids, as described in Isler et al., Helv. Chem. Acta, (1956), No. 237 page 2041, is as laborious as it is non-selective, as HIP must be provided as a pure substance and rearrangement to DH-KIP in the presence of strong acids entails the formation of trimethylphenols as by-products. A reaction time of 20 h is also a substantial disadvantage of this process.
Hitherto, there has been no known process which allows DH-KIP to be produced using inexpensive, industrially efficient and accessible oxidizing agents and high product selectivity, in particular from a readily available mixture of xcex2-isophorone epoxide and hydroxyisophorone. This is the basis of the objects to be achieved by the invention.
An object of the present invention is to produce dihydroketoisophorone (DH-KIP) from xcex2-isophorone with high selectivity and yield, suppressing in particular the re-isomerization of the educt to alpha-isophorone, to avoid having to separate alpha-IP from the product solution by expensive means and having to change the environment from acid (environment for epoxidation with organic percarboxylic acids) to basic (prior art for the production of HIP from xcex2-IPO) and back to acid (prior art for rearrangement of HIP to DH-KIP). Furthermore a process is to be provided, which makes it possible to convert the mixture of xcex2-IPO and HIP obtained xe2x80x9cin situxe2x80x9d directly to DH-KIP and to reduce considerably the long reaction times in the prior art.
A further object of the invention is to provide a process for converting xcex2-isophorone epoxide to DH-KIP, xcex2-isophorone epoxide with its isomer hydroxyisophorone being produced from xcex2-isophorone in a first reaction step by reaction with an organic peracid.
The above and other objects can be achieved in that xcex2-isophorone is brought into contact at moderate temperatures with an organic percarboxylic acid dissolved in an organic solvent, the organic solvent being at the same time the extracting agent used to extract the organic peracid as it is produced, and the xcex2-IPO thus obtained together with its isomer hydroxyisophorone which is present as a by-product in a concentration of 0.1-80 mol % (in relation to xcex2-IPO) is isomerized to DH-KIP by thermal isomerization in the presence of the organic carboxylic acid or by reaction in the presence of a salt as catalyst.
According to the process, inexpensive raw materials such as aqueous hydrogen peroxide, organic carboxylic acids and a solvent on the one hand and, in the simplest case, alkali- or earth alkali salts on the other, can be used, and as only the educt substrate xcex2-isophorone and hydrogen peroxide are spent during the epoxidation process, the majority of the other raw materials can be recycled. The catalysts used for the isomerization of xcex2-IPO/HIP solutions can also be recycled and in this sense are not specific consumption materials.
The first aspect of the invention relates to a new process for the production of dihydroketoisophorone by epoxidation of xcex2-isophorone (xcex2-IP) by reaction with a peracid in the form of its solution in a suitable solvent in the liquid phase, which is stable under reaction conditions, thus substantially avoiding separation into two phases which can occur if there is a high concentration of water in the reaction mixture, and obtaining a mixture of organic carboxylic acid, extracting agent (=solvent), xcex2-isophorone epoxide with hydroxyisophorone as the product solution. In the simplest form of the process according to the invention, the solvent used for oxidation is also the agent for extracting the corresponding peracid as it is produced from an aqueous hydrogen peroxide solution, carboxylic acid and sulfuric acid.
The second aspect of the process according to the invention relates to the rearrangement of the xcex2-IPO formed xe2x80x9cin situxe2x80x9d, which, depending on the production process, may contain 0.1-80 mol% hydroxyisophorone, by direct thermal isomerization of the solution obtained from epoxidation or isomerization of the epoxidation solution obtained directly from the reaction in the presence of an additional catalyst salt, in the simplest case an alkali- or earth alkali salt or other suitable isomerization catalysts, or by isomerization of an epoxidation solution, the majority of which at least is first released from the carboxylic acid by extraction or distillation.
To achieve high selectivity, the reaction is carried out in two temperature stages.
In particular, the present invention is a process for the production of dihydroketoisophorone (2,2,6-trimethylcyclohexane-1,4-dione) 
by reacting xcex2-isophorone (3,5,5-trimethylcyclohex-3-en-1-one) 
with an organic percarboxylic acid in the form of its non-aqueous solution in an inert organic solvent at a temperature of 0xc2x0 C. to 300xc2x0 C., the percarboxylic acid used as the oxidizing agent being formed in a pre-determined equilibrium and absorbed or extracted with the solvent used for oxidation, characterized in that, the mixture of the xcex2-isophorone epoxide formed first 
and the isomer 4-hydroxyisophorone (HIP), 
is isomerized xe2x80x9cin situxe2x80x9d thermally or in the presence of an acid catalyst or in the presence of a salt.
Epoxidation of xcex2-IP takes place at temperatures of xe2x88x9250xc2x0 C. to 100xc2x0 C. and in a preferred embodiment at temperatures of 0xc2x0 C. to 60xc2x0 C. The reaction times in this temperature range are fast enough when used as an industrial process, to achieve complete conversion of xcex2-isophorone within 0.5-5 h. For safety reasons, a method can also be chosen which uses an excess of xcex2-isophorone in relation to the percarboxylic acid used, but the stoichiometric consumption of percarboxylic acid at moderate temperatures is fast enough, even with this variant.
Although the reaction can also be carried out at higher temperatures, this measure increases the proportion of hydroxyisophorone. High product selectivities can be achieved at lower temperatures, but the reaction speed is reduced and a cooling brine must be used.
Isomerization of the intermediate product mixture from epoxidation (consisting of xcex2-IPO and HIP) takes place at temperatures of 0xc2x0 C.-300xc2x0 C., and depends substantially on the desired reaction time, the catalyst used and its concentration. In a preferred embodiment the working temperature is 20xc2x0 C. to 200xc2x0 C. Reaction times in this temperature range are fast enough when used as an industrial process to achieve complete conversion of xcex2-IPO/HIP to DH-KIP within 0.5-3 h.
In a further embodiment of the process according to the invention, epoxidation and isomerization are carried out simultaneously in one temperature range, the presence of the isomerization salt during epoxidation being a characteristic of this embodiment. According to a simple variant of this embodiment, xcex2-IP is provided with or without solvent, in the presence of the salt catalyst, e.g. MgBr2, and percarboxylic acid is added to the organic solvent at temperatures of 20xc2x0 C.-150xc2x0 C.
The ratio of xcex2-isophorone to the peracid used as the oxidizing agent is not critical for product selectivity and, depending on the method used, can be selected in a molar ratio of xcex2-IP:percarboxylic acid=10:1 to 1:10. A preferred molar ratio of the components is in the range 2:1 to 1:2. In view of the potential risks of working with peracids it is possible to use slightly less of the epoxidizing agent in relation to the xcex2-isophorone, so that when the reaction is complete, the peracid has been converted, forming the corresponding carboxylic acid and only a slight concentration of excess xcex2-isophorone is present. The unconverted xcex2-isophorone can either be re-used as it is for preparation or, after isomerization to alpha-IP, can be fed back into the xcex2-IP production process.
The concentration of peracid in the extracting agent or the solvent used for oxidation can be 1 wt.-% to 50 wt.-%, the preferred range on safety grounds being 5 wt.-%-30 wt.-%. The maximum concentration range of peracid in the organic solvent to be selected must in particular depend on the choice of peracid used, the solvent and the oxidation temperature.
A plurality of commercial organic solvents can be used as the oxidizing solvent or as the agent for extracting the peracid.
The xcex2-isophorone epoxide obtained at the end of the reaction can be further converted under suitable conditions directly after isolation. In particular, after isolation, xcex2-IPO can be converted by known processes to hydroxyisophorone or to 2,2,6-trimethyl-cyclohexane-1,4-dione (DH-KIP=dihydroketoisophorone). The isolation of xcex2-IPO can be avoided in the process according to the invention, by converting the organic mixture of xcex2-IPO and HIP directly in the presence of a suitable catalyst, in particular a Lewis acid or various salt catalysts.
The organic carboxylic acid used for the present process is preferably an aliphatic, alicyclic, aromatic carboxylic acid having 1-20 hydrocarbon atoms, wherein the hydrocarbon group may have one or more functional groups. Formic acid, acetic acid, propionic acid, butyric acid, valeric acid and higher homologues are preferred as percarboxylic acid-forming carboxylic acids. Particularly preferred are non-aqueous solutions of performic acid, peracetic acid, perpropionic acid and homologous peracid compounds having 1-20 hydrocarbon atoms. Examples of the branched derivatives which may be used are isobutyric acid, pivalic acid and neopentyl carboxylic acid. Examples of aromatic, substituted and unsubstituted percarboxylic acid-forming carboxylic acids are benzoic acid, m-chloro- and p-nitrobenzoic acid, and also monoterephthalic acid. Halogenated derivatives such as for example trihalogenated acetic acid (trichloro-, trifluoroacetic acid) are also suitable.
The production of the peracid from the corresponding carboxylic acid is known per se. It is explained in one embodiment by the example of the formation of peracetic acid. Acetic acid is normally reacted with aqueous hydrogen peroxide in the presence of an acid catalyst (in the case of carboxylic acids activated by corresponding substituents which have sufficient acidity, there is no need to supplement with additional catalyst acids. Formic acid also forms performic acid xe2x80x9cin situxe2x80x9d in the presence of hydrogen peroxide without the need for the addition of a mostly mineral catalyst acid).
Normally sulfuric acid is used as the catalyst acid. In the present invention, the peracid used as an oxidizing agent is preferably formed in a pre-determined equilibrium reaction from the corresponding carboxylic acid, an aqueous hydrogen peroxide solution and sulfuric acid and absorbed and extracted with the solvent used for oxidation and isomerization. Mixing carboxylic acid/aqueous hydrogen peroxide/sulfuric acid produces more or less pure aqueous solutions of the peracetic acid and, after establishing equilibrium, a so-called equilibrium peracetic acid of the following composition (Ullmans Encyclopaedia of Technical Chemistry, 3rd edition, vol. 13, p. 254):
peracetic acid: approx. 40-42%
acetic acid: approx. 37-40%
water: approx. 10-14%
H2O2: approx. 4-6%
sulfuric acid: approx. 0.5-1%
Very clean aqueous solutions are obtained if, as well as the catalyst acid and water, a mol quotient H2O2:peracid of  greater than 1 is added and after equilibrium has been established the peracid is removed from the top of a distillation column as an azeotrope with water at reduced pressure. According to DE 11 65 576, anhydrous solutions of the peracid in an organic medium can be produced from these solutions. The anhydrous or water-depleted solution of the peracid in an organic medium can also be produced from the reaction mixture H2O2xe2x80x94catalyst acidxe2x80x94carboxylic acid by azeotropic distillation of the water with a suitable solvent (see Ullmans Encyclopaedia of Technical Chemistry, suppl. volume, 3rd edition, p. 181). A further method which, with regard to the safety risks associated with handling these solutions, can be classified as comparatively safe, is to extract the peracid directly from the aqueous solution containing the percarboxylic acid, using a suitable extracting agent.
It has been found that the isomerization can also be catalyzed by the carboxylic acid formed from reduction of percarboxylic acid.
Suitable extracting agents for the purposes of the invention are described in DE 21 45 603, including aliphatic, cycloaliphatic, aromatic solvents, but halogenated derivatives of compounds from these classes of substances are also suitable, in particular chlorinated hydrocarbons such as methylene chloride and chloroform. The hydrogen peroxide solutions used may have an H2O2 content of 10-90 wt.-%, commercial aqueous peroxide solutions with a content of 30-85 wt.-% being used in particular, and preferably a solution with a content of 45-70 wt.-%.
An equally suitable solution for the epoxidation of xcex2-isophorone can be produced from ripened solutions of percarboxylic acids by extraction with phosphoric acid esters, in particular trialkylphosphates according to U.S. Pat. No. 3,829,216. Here absorption is first carried out with the stated phosphoric acid esters and then the 10-80 wt.-% peracid solutions are desorbed with an alkyl ester, to form, finally, solutions of the peracid in an alkyl ester.
In a preferred embodiment of the process according to the invention, the water-depleted solution of the organic percarboxylic acid is produced according to U.S. Pat. No. 4,904,821. This patent describes the production of a percarboxylic acid solution in alkyl phosphates, by placing H2O2 (30-35 wt.-% aqueous solution) and acetic acid in a molar ratio of 1 to 2:1 in the bottom of a distillation column, setting the quantity of sulfuric acid to 20-30 wt.-% in relation to the volume of the total solution and removing a mixture of acetic acid, peracetic acid and water from the top at temperatures of 55xc2x0 C. to 70xc2x0 C. at reduced pressure. The vapors are absorbed in a suitable phosphoric acid trialkylester while non-absorbed water can be removed from the top of this absorption column.
The solutions of the percarboxylic acid along with the corresponding carboxylic acid in the phosphoric acid ester are then used directly for epoxidation, or the solutions are converted to another conventional solvent by desorption.
The contents of percarboxylic acid solutions in the solvent or in the extracting agent are normally 1-50 wt.-%, aiming preferably for a dilution of  less than 30 wt.-%, in order to ensure safe handling.
The process according to the invention can be carried out as described in the presence of organic solvents which are inert under reaction conditions. The concentration of the reagent in the solvent has little influence on the product profile of the reaction and is substantially dependent on safety aspects. A solvent-free method can also be used, in which case xcex2-isophorone is brought directly into contact with the solution containing the peracid, if it can be ensured that the water concentration of the solution containing the peracid is low enough to suppress substantially both re-isomerization to alpha-IP and consecutive reaction of the epoxide to HIP. The water concentration of the organic solution of percarboxylic acid used should normally be lower than 5 wt.-%, and is preferably 0.01 to 2 wt.-%.
The solvents used must normally be stable in the presence of peracids. A further selection criterion is that they should be easy to separate by distillation from the carboxylic acid used and from the product.
If epoxidation is carried out in the presence of organic solvents, it is advantageous to use aliphatic and cyclic esters, for example acetic acid methylester, acetic acid ethylester, acetic acid propyl ester, butyl acetate, isobutyl acetate, gamma butyrolactone, ethylene carbonate, derivatives and homologues thereof, aliphatic, alicyclic and aromatic hydrocarbons, for example pentane, hexane, heptane, octane and other homologues, benzene, toluene and xylene. Ketones such as for example acetone, methyl ethyl ketone, diethyl ketone and isophorone are also suitable as solvents within the scope of the invention. Aliphatic, aromatic or mixed ethers such as diethyl ether, methyl tertiarybutyl ether can also be used, although their use is limited for safety reasons. Suitable organic phosphoric acid esters are those of which the substituents contain 3-30 hydrocarbons, for example phosphoric acid triester, the substituents of which can be of an aliphatic, alicyclic or aromatic nature. Examples of these are tricycylohexylphosphate, triphenylphosphate tricresylphosphate, diphenylcresylphosphate, triethylphosphate, tributyl-phosphate, trioctylphosphate and other derivatives and homologues.
Other suitable classes of solvent are halogenated hydrocarbons. Mixtures of these solvents can also be used.
The rearrangement of the mixture of xcex2-isophorone epoxide and hydroxyisophorone obtained xe2x80x9cin situxe2x80x9d which, depending on the parameters set for epoxidation, has a concentration of 0.1-80 mol % in relation to xcex2-IPO, can in principle be carried out by various methods.
As a one-pot process from xcex2-IP to DH-KIP without intermediate isolation of xcex2-IPO/HIP solutions: According to this variant epoxidation of xcex2-IP is carried out in the presence of an isomerization catalyst, which catalyzes the conversion of both xcex2-IPO and HIP to DH-KIP.
According to the indirect process variant, epoxidation is carried out first and, after separation from the carboxylic acid, the xcex2-IPO/HIP product mixture is then converted to DH-KIP by the addition of isomerization catalysts.
The main catalysts for isomerization of xcex2-IPO and HIP to DH-KIP are Brxc3x6nstedt- or Lewis acids, but various salts, in particular alkali- and earth alkali salts, which have no exceptional acidity, can be used.
The Brxc3x6nstedt acids which can be used according to the invention as isomerization catalysts, are mainly protonic acids with a pKA value of less than 10. This covers relatively mild catalyst acids in a pKA range of pKA=3-10 such as carbonic acid, ammonium or pyridinium salts, boric acid or common organic carboxylic acids (substituted or unsubstituted) such as acetic acid, propionic acid or chloroacetic acid and similar compounds. Di- or tricarboxylic acids such as oxalic acid, malonic acid, adipic acid and higher homologues or citric acid and related compounds can be used as carboxylic acids.
Other more active isomerization catalysts are protonic acids with a pKA less than 3 of the group sulfuric acid, hydrohalic acid (HCl, HBr, HI), fluoroboric acid, aromatic sulfonic acids such as p-toluene sulfonic acid or benzene sulfonic acid, trifluoroacetic acid, chloroacetic acid or corresponding aromatic carboxylic acids, which are activated by electron-attracting groupings such as picric acid, nitroterephthalic acid and derivatives.
Active Lewis acids used as isomerization catalysts according to the invention are, for example, compounds of the aluminum halogenide group (AlCl3, AlBr3, AlF3) boron trifluoride, iron-(III)-halogenide, zinc halogenide, tin halogenide and titanium tetrachloride. Corresponding acetylacetonates may also be used for the isomerization of xcex2-IPO and HIP at corresponding temperatures.
Active alkali- and earth alkali salts used as isomerization catalysts according to the invention are for example the alkali- or earth metal alcaline halogenide compounds of groups IB, IIB and VIIIB or corresponding Lewis acids known to the person skilled in the art. Examples of alkali- or earth metal alcaline halogenides are LiCl, NaCl, KCl, MgCl2 MgBr2, or CaCl2.
Most preferably, the mixture obtained as an intermediate is isomerized in the presence of a solid, acid or super acid catalyst and the reaction proceeds as a solid/liquid two-phase process.
The quantity of catalyst in relation to the isomerizing substrate mixture xcex2-IPO/HIP varies greatly depending on the type of catalyst used. In general very good yields can be achieved with adequate space-time yields when using 10xe2x88x922 mol% to 1000 mol%, preferably between 10xe2x88x922 mol % to 50 mol % of catalyst. In principle, when using very acid isomerization catalysts (strong Brxc3x6nstedt acids or super acids with Ho greater than xe2x88x9211.9), very low catalyst quantities of 10-2 mol % to 5 mol % are sufficient. When using Lewis acids, which are at least partially soluble in the reaction mixture, a catalyst concentration of 10xe2x88x921 mol % to 5 mol% is normally sufficient and with heavy soluble salts catalyst concentrations are normally set at 1 mol % to 50 mol % to ensure adequate reaction times. When using heterogeneous catalysts, either the salt catalyst can be used in suspension or the process can be carried out by the fixed bed method.
In one embodiment of the process according to the invention, the xcex2-isophorone epoxide formed is isolated directly after elimination of excess peroxide and peracid. On a laboratory scale it is also possible to shake with a little aqueous sodium bisulphite solution until the result of the peroxide test is negative. When working with excess xcex2-IP, it is necessary to ensure, once the reaction is complete, that no excess peracid is left in the product mixture. Once the reaction is complete and peroxide impurities have been destroyed a product mixture remains which consists substantially of carboxylic acid, xcex2-IPO, hydroxyisophorone and the solvent. The carboxylic acid can be separated off by fractionation and thus recycled. A further possibility for separating the organic carboxylic acid is extraction with a polar extracting agent, which is immiscible with the organic phase and inert towards the product. In the simplest case, the carboxylic acid formed can be extracted with a little water.
After separating off the carboxylic acid a mixture of xcex2-IPO and HIP remains in the solvent of the epoxidation reaction, which can usefully also be used as a solvent for the subsequent isomerization. After isomerization the isomerization catalyst can be separated off by conventional separation processes such as distillation or extraction or simple centrifugation in the case of non-soluble, heterogeneous suspension catalysts.
Before isomerization, intermediate isolation of xcex2-IPO may also be carried out by distillation, the solvent normally constituting the low-boiling component in relation to xcex2-IPO. When using high-boiling absorbents, extracting agents or solvents such as the phosphoric acid triesters mentioned above, xcex2-IPO can also be the volatile component. The remaining reaction medium can be fed back into the reaction after separation from the product.
After isomerization the DH-KIP obtained, which is normally dissolved in the organic solvent, can be isolated by suitable methods. A high degree of product purity is obtained by crystallization or distillation.
The purity of the dihydroketoisophorone isolated in this way corresponds to the product quality required for use as an educt for synthesis of trimethylhydroquinone (TMHQ) and trimethylhydroquinonediacetate (TMHQ-DA), the intermediates for synthesis of vitamin E acetate.